A user interface is considered as an interface, via which a user can interact with a device the user is operating. The user interface typically consists of an input device, such as a keyboard, by which the user controls the functions of the device, and an output device, such as a display, for graphically indicating the user about the outcome of the control. In a graphical environment, it is customary to use mechanical pointing devices, such as a mouse or a trackball, for moving a pointer on the display. The mechanical pointing devices are, however, relatively large in size and unreliable due to exposure to mechanical stress and dirt.
Optical user interfaces are also suggested, which are based on using the user's finger to control a pointing device. Such interfaces include a scanning surface, on which the user's finger is placed. The scanning surface is illuminated, and an image of the fingerprint is recorded with a camera placed underneath the scanning surface. The size of the scanning surface is selected such that it substantially exceeds the contact area generated when the user presses his finger against the scanning surface.
The prior art optical user interfaces are, however, not designed for a portable electric device, such as a mobile phone, where small battery consumption and low manufacturing costs are required. In prior art optical user interfaces, the scanning surface is relatively large, and therefore, the required optical power of the light source illuminating the scanning surface as well as the size of the camera forming the image of the fingerprint cause high costs and substantial power consumption. Therefore, it is desirable to consider alternative techniques for providing optical user interfaces.